The present invention relates generally to the measurement or detection of magnetic fields and, more particularly, to a system, apparatus, and method that utilizes a sensor positioned inside a wellbore casing to measure the magnetic field therein. Alternatively, the present invention relates to the measurement or detection of a magnetic field inside a wellbore casing to determine a property or characteristic of the wellbore casing or wellbore environment.
Subsurface characterization of geologic formation is an important aspect of the drilling of oil and gas wells. Subsurface characterization helps to identify, among other things, the structure and fluid content of the geologic formations penetrated by a wellbore. The formation may contain, for example, hydrocarbon products that are the target of drilling operations. Geologic formations that form a hydrocarbon reservoir contain a network of interconnected fluid paths, or xe2x80x9cpore spaces,xe2x80x9d in which, for example, hydrocarbons, water, etc., are present in liquid and/or gaseous form. To determine the hydrocarbon content in the pore spaces, knowledge of formation characteristics such as porosity and permeability is often required or at least helpful.
Information about the geologic formations and about reservoir characteristics promote efficient development and management of hydrocarbon resources. Reservoir characteristics include, among others, the resistivity of the geologic formation containing the hydrocarbons. Resistivity is generally related to porosity, permeability, and fluid content of the reservoir. Because hydrocarbons are generally electrically insulating and most formation water is electric conductive, formation resistivity (or conductivity) measurements are valuable exploration tools. Moreover, formation resistivity measurements may be used to monitor changes in reservoir hydrocarbon content during production of hydrocarbons.
In many subsurface geophysical exploration techniques, a probe having sensors for measuring various physical parameters at different depths is lowered into a wellbore. In one type of measurement, a magnetic field sensor is used to measure induced magnetic fields and thereby glean information regarding the possible presence of conductive ore bodies. Associated with the magnetic field sensor is a transmitter coil that, when energized, establishes a magnetic field. The magnetic field induces electrical currents in conductive regions, and the induced currents establish a magnetic field that is then measured. By performing such measurements at various depths, it is possible to establish field profiles.
In certain applications, the transmitter coil is a large horizontal loop of wire that is laid out on the surface of the earth in the general vicinity of the wellbore. In other applications, the transmitter coil is a small diameter coil that is lowered into the wellbore, and may be physically located in the same housing as the sensor.
The performance of a magnetic field sensor or magnetic receiver positioned within a wellbore casing may be compromised by the conductive casing""s effect on the magnetic field to be measured. Specifically, the measurable magnetic field induces a current that flows concentrically about the receiver coil and tends to reduce the magnetic field within the casing. As will be further explained in the Detailed Description, the measurable magnetic field may be highly attenuated as a result and the measurements made by the receiver may be influenced by variations in attenuation caused by variations in the conductive casing""s properties. Often, the design criteria for a crosswell survey of a cased wellbore reduces the magnetic field signal to a level that is undetectable by standard receivers. Moreover, the variance in conductivity, permeability, and thickness along a longitudinal axis of a length of casing makes it difficult to determine an attenuation factor at any selected point. The inability to determine an attenuation factor at a selected point along the casing may cause errors in field measurements that are not easily corrected.
One prior attempt to overcome the adverse effect of the casing on the receiver measurements involves inclusion of a separate small-scale transmitter-receiver within the cased wellbore to measure the casing properties. The measured casing properties are then used to correct the measured crosswell data. See, e.g., Lee et al., Electromagnetic Method For Analyzing The Property of Steel Casing, Lawrence Berkeley National Laboratories, Report 41525, February, 1998.
Another prior attempt to correct or account for the magnetic field attenuation involves positioning a monitor receiver adjacent the transmitter in the cased wellbore. In this manner, the attenuation sensed by, for example, a receiver located in an adjacent wellbore may be predicted. This method is disclosed in U.S. patent application Ser. No. 09/290,156, filed Apr. 12, 1999, entitled Method and Apparatus for Measuring Characteristics of Geologic Formations, and assigned to the assignee of the present invention (hereby incorporated by reference and made a part of the present disclosure).
The present invention relates to a magnetic receiver with a magnetically permeable core that is positioned inside a wellbore casing to measure or otherwise detect the magnetic field therein. The receiver employs or includes a feedback means to reduce effect of the casing on the measurable magnetic field (xe2x80x9cthe casing effectxe2x80x9d), i.e., reducing the mutual coupling between the core and the casing, thereby reducing the attenuation of em signals otherwise seen inside the casing. In one embodiment, the feedback means is provided in the form of an extra or feedback winding that reduces the mutual coupling between the core and the casing, thereby effectively reducing or canceling the inducing field inside the core and reducing the attenuation otherwise caused by the core-casing interaction. In an alternative embodiment, the magnetic receiver employs a current feedback amplifier circuit to cancel the inducing field inside the core. Specifically, the circuit is operated to produce a secondary magnetic field that essentially cancels the inducing field inside the core.
As used herein, and for the purpose of fully describing the inventorsxe2x80x9d contribution to the art , the terms xe2x80x9cmeasurexe2x80x9d (or xe2x80x9cmeasurementxe2x80x9d) and xe2x80x9cdetectxe2x80x9d (or xe2x80x9cdetectionxe2x80x9d) are synonymous and each shall incorporate the meaning and scope of the other. Similarly, the term xe2x80x9creducexe2x80x9d shall incorporate the meaning of xe2x80x9ccancelxe2x80x9d with respect to the reduction or cancellation of a magnetic field or casing effect.
In one aspect of the invention, a method of measuring a magnetic field inside a conductive wellbore casing is provided, wherein electromagnetic energy propagates through the casing at a frequency of over about 1 Hz. Such a method includes providing a magnetic receiver having a permeable core and a main winding wound about the core, and incorporating a feedback means with the magnetic receiver. The magnetic receiver is then positioned inside the wellbore casing. Further, electromagnetic energy is generated (e.g., from a transmitter in another wellbore casing) externally of the casing and directed so as to propagate through the casing at a frequency above 1 Hz (and more preferably between about 1 Hz and about 1000 Hz), thereby creating a total magnetic field inside the casing consisting of a primary inducing field and a secondary field inside the casing that generally opposes the primary inducing field. The receiver is then operated in a feedback mode to cancel or reduce at least a portion of the total magnetic field and then to measure the total magnetic field.
In one embodiment, the feedback means includes a secondary winding positioned about the core, and the method includes amplifying the output of the main winding and feeding the amplified output back through the secondary winding. The output is amplified and fed such that a core flux generated by the second winding is generally in opposition to a core flux generated by an external field, thereby substantially canceling the flux in the core. Further, the amplified output is fed back such that a feedback current is driven through the feedback winding and is generally linearly proportioned to the secondary magnetic field.
In another respect of the invention, a method is disclosed for reducing attenuation of the magnetic field that is otherwise caused by a mutual coupling of the core with the casing. This inventive method is employed in conjunction with or in relation to a method of employing a magnetic receiver positioned within a conductive casing to measure an inducing magnetic field therein, wherein the receiver has a permeable core and a main winding thereabout.